1. Field of the Invention
The present invention relates to a catalyst for polymerization of olefins exhibiting excellent catalytic activity to hydrogen and capable of maintaining high stereoregularity of polymers and a high yield.
2. Description of Background Art
A solid catalyst component containing magnesium, titanium, an electron donor compound compound, and a halogen as essential components used for polymerization of olefins such as propylene has been known in the art. A large number of methods for polymerizing or copolymerizing olefins in the presence of a catalyst for olefin polymerization comprising the above solid catalyst component, an organoaluminum compound, and an organosilicon compound have been proposed. For example, Patent Document 1 (Japanese Patent Application Laid-open No. 57-63310) and Patent Document 2 (Japanese Patent Application Laid-open Japanese Unexamined Patent Publication No. 57-63311) propose a method for polymerizing olefins, particularly olefins with three or more carbon atoms, in which a catalyst comprising a solid catalyst component containing a magnesium compound, a titanium compound, and an electron donor compound, an organoaluminum compound, and an organosilicon compound having an Si—O—C linkage in combination is used. However, because these methods are not necessarily satisfactory for producing highly stereoregular polymers in a high yield, improvement of these methods has been desired.
Japanese Patent Application Laid-open No. 63-3010 proposes a catalyst and a method for polymerizing propylene. The catalyst comprises a solid catalyst component, obtained by processing a powder produced from dialkoxymagnesium, aromatic dicarboxylic acid diester, aromatic hydrocarbon, and titanium halide with heat, an organoaluminum compound, and an organosilicon compound.
Japanese Patent Application Laid-open No. 1-315406 proposes another propylene polymerization catalyst and a method for polymerizing propylene in the presence of this catalyst. The catalyst comprises a solid catalyst component obtained by preparing a suspension from diethoxymagnesium and alkyl benzene, causing this suspension to come into contact with titanium tetrachloride, reacting the resulting product with phthalic acid dichloride to produce a solid product, and further reacting the solid product catalytically with titanium tetrachloride in the presence of alkyl benzene, an organoaluminum compound, and an organosilicon compound.
All of the above-described conventional technologies have attained certain results in improving catalytic activity to the extent of permitting dispensing with an ash-removal step for removing catalyst residues such as chlorine and titanium from formed polymers, improving the yield of stereoregular polymers, and improving durability of catalytic activity during polymerization.
The polymers produced using these catalysts are used in a variety of applications including formed products such as vehicles and household electric appliances, containers, and films. These products are manufactured by melting polymer powders produced by polymerization and forming the melted polymers using various molds. In manufacturing formed products, particularly, large products by injection molding or the like, melted polymers are sometimes required to have a high fluidity (a melt flow rate). Accordingly, a large number of studies have been undertaken to increase the melt flow rate of the polymers.
The melt flow rate greatly depends on the molecular weight of the polymers. In the industry, hydrogen is generally added as a molecular weight control agent for polymers during polymerization of propylene. In this instance, a large quantity of hydrogen is usually added to produce low molecular weight polymers having a high melt flow rate. However, the quantity of hydrogen which can be added is limited because pressure resistance of the reactor is limited because of safety. In order to add a larger amount of hydrogen, the partial pressure of monomers to be polymerized has to be decreased. The decrease in the partial pressure, however, is accompanied by a decrease in productivity. Additionally, use of a large amount of hydrogen may bring about a problem of cost. Development of a catalyst capable of producing polymers with a high melt flow rate by using a smaller amount of hydrogen, in other words, a catalyst which has a high activity to hydrogen as compared with conventional catalysts, while maintaining capability of producing polymers with a high stereoregularity and exhibiting high yield performance has therefore been desired. Conventional technologies have been insufficient in solving these requirements.